As a professional autograph seeker/collector, let me ease some of your minds. Let me tell you how these things mostly work.
First, Yes, Tom and Rupert will be at the event. Unless something prevents them from going, they've both agreed to do it last i heard.
I was told when booking the premiere package (we were told it closed out while we were on the phone cause before we hung up we asked to add my son to one and they said no it's sold out). So just the two old fogies here (I am 50 and my hubby is 52).
Alas, they said to us on the phone that there will be more than one actor/crew member at the autograph session (probably three per hour session). It could change as we all know, but I am guessing you'll at least get three signatures. I just don't understand if they only have less then 1000 people, why they just don't line all the celebs up and let us go through the line all at once. Or have two sessions. I am sure they would agree to two hours or so worth of work. I've been in a great many lines at various bookstores, events and conventions as well as hanging around the backstage door at concerts etc. I know how it goes.
You most likely will go to a room (not outside in a tent unless it's a roped off area, as people will be in the park who will try to get in). My guess is it'll be in a building. You'll line up, the celeb will be at a table. There will be one autograph on a supplied image (probably something cheesy supplied by the park like a cheap b/w 5x7 like Disney does for Star Wars Weekends). that's one of your items. You are allowed ONE item of your own. Period. So if three celebs are there, you'll get the same three sigs on one item (whatever you bring) plus one photo of each. Hope there's more than that but don't expect it just in case!
Then, you will be allowed to take photos OF but not with the celebs in order to save time. But I've seen time and again, the rules change. So it's not good for you all to speculate. Cause even if they say NO now, it's not up0 to them in the end. It's up to the celeb half the time. It just depends on if they have say a news interview (like Catherine Bach did on one event I recently attended), or if they have to MC something like some of the Star Wars people did at Disney. If not, then they will stay, till everyone has their autographs and if they want to be fan friendly, they'll take photos of it. You better pray there is only 200 total sets sold, or you will not get your photo with them most likely due to time. The less people at this event, the better.
As for the gala. This is NOT going to be a ball or anything. It's probably park food in the HP area and it's probably just free access to the park with less crowds; possibly, they'll have some kind of something with the celebs. Either some mingling or some small talk on a stage for a couple minutes etc. It's not going to be where you dress up, because we were told "park attire" as in wear jeans and t-shirts like you would do in the park. I plan to at least look clean and neat though!
I heard also that there may be one or two extras. Not sure what that is. Could be something as innocuous as a free poster or maybe extra time in the park, or could be just a free ice cream cone! I would expect small and if it's better, you'll be really excited then. Just my opinion.
But don't speculate about how the autograph session will run. I've seen professional conventions run extremely well go awry. You have to understand the celeb has some control, warner brothers has some control and there's the middle people stuck in the middle who do not know who to listen to (who's the boss) and make up THEIR own rules! So what is said today might not be that next week or the day or hour you go. Heck, I was in a convention line where I was in the first 10 people and by the time they got to me, the total amount of items, personalization etc had changed THREE times! And there were THOUSANDS in line and after 30 min, the celeb got up and decided that was enough even though they agreed for 8 hours.... and I've seen the promoters say no photos and Catherine Bach stayed there 10 hours because she insisted on photos with everyone; and that cause hundreds to be without an autograph... you just never know! I've been in author lines where they rushed people through and the author was sitting there with nothing to do for the rest of the time and they'd of been better off letting them take photos!
It's all a crapshoot. Just expect the least possible and pray for the best.
take a look at my Potter autographs (biggest collection I know of so far): starwarsautographcollecting.com